The invention relates to nonlinear resistive valve elements of surge arresters, and more particularly to an electrical insulating coating therefor.
Surge arresters are used in electrical power systems for limiting surge voltages on the power lines and the equipment connected thereto by discharging or bypassing surge current to ground. These surge arresters include a series gap which normally insulates the surge arrester from the power system, but which sparks over when the voltage across the series gap becomes excessive. Valve elements, connected between the series gap and ground, are used to limit the power follow current subsequent to a drop in the impressed voltage so that the series gap can then interrupt the follow current and again insulate the surge arrester from the power system. To perform this function, the valve element must have the capacity to discharge high currents at excessive voltages while increasing its resistance to discharges at normal voltages.
In a cylindrical shaped valve element such as that disclosed in U.S. Pat. No. 3,813,296 issued May 28, 1974, to Darrell D. McStrack and James E. Schroeder, the valve element is composed of silicon carbide particles bound together within a ceramic matrix. During voltage surges the silicon carbide particles are subjected to high voltage stresses which can cause ionization of the air in contact with the particles. If the insulation strength of the air between the particles is exceeded, an arc discharge will occur. Accumulation of these discharge arcs can cause a complete valve element flashover across the outer cylindrical surface of the valve element. Once the valve element has flashed over, the valve element is ineffective in limiting the power follow current. Consequently the series gaps cannot interrupt the high power follow current, and failure of the surge arrester results.
To prevent such flashover failure of the surge arrester, it is necessary to apply an electrically insulating coating over the outside surface of the valve element, which is free from pin holes and completely bonded to the valve element body so that the area between the body and the coating is free of voids through which arcs can travel. In order to meet the requirements of the American National Standard, A.N.S.I C62.1-1975, entitled "Surge Arresters for Alternating Current Power Circuits", this insulating coating must not only have a high electrical breakdown strength, but also must have good mechanical strength and high resistance to cracking to withstand the sudden pressure exerted on it while a surge current is flowing through the valve element caused by expanding gases within the valve element. Also it must retain these mechanical and electric properties over a wide temperature range, from the lowest ambient temperature expected to be encountered at the locations where surge arresters are used, to a peak high temperature of the valve element and coating caused by repeated operations of the surge arrester within a relatively short period of time.
In the past, ceramic electrically insulating coatings, such as that described in the above-referenced U.S. Pat. No. 3,813,296, have been used to insulate the cylindrical sides of valve elements of silicon carbide particles bound together within a ceramic matrix. Generally, the materials comprising the ceramic coating are mixed together in suitable milling apparatus, such as a ball mill, to improve the smoothness and decrease the porosity of the final coating, then degassed to remove any air entrapped during mixing and thus minimize potential voids in the ceramic coating. Next, the ceramic coating mixture is applied to the valve element before the valve element has been fired to cure its ceramic matrix. The coated valve element is then fired at an elevated temperature to mature both the ceramic matrix of the valve element and the ceramic forming ingredients of the coating mixture.
While the single firing procedure of the coated valve element reduces the time required to manufacture these valve elements, it does not allow the valve valve elements to be inspected after firing for cracks which can develop during firing underneath the insulating coating. Also, an insulating coating which can be more easily prepared and applied to the valve element than known ceramic coatings would be highly desirable.